Supramolecular sandwiches: Halogen-bonded coformers direct [2+2] photoreactivity in two-component cocrystals

Article abstract of DOI:10.3390/molecules25040907

The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene (1,3-di-I-tFb and 1,4-di-I-tFb, respectively) form cocrystals with trans-1,2-bis(2-pyridyl)ethylene (2,2′-bpe) assembled by N···I X-bonds. In each cocrystal, 2(1,3-di-I-tFb)·2(2,2′-bpe

Full text of DOI:10.3390/molecules25040907

Article
Supramolecular Sandwiches: Halogen-Bonded
Coformers Direct [2+2] Photoreactivity in
Two-Component Cocrystals
1
1
1,
Jay Quentin , Dale C. Swenson and Leonard R. MacGillivray *
Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA; jay-bell@uiowa.edu (J.Q.);
dale-swenson@uiowa.edu (D.C.S.)
* Correspondence: len-macgillivray@uiowa.edu
Academic editor: Gianluca Ciancaleoni
Received: 18 January 2020; Accepted: 11 February 2020; Published: 18 February 2020
Abstract: The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene (1,3-di-I-tFb
and 1,4-di-I-tFb, respectively) form cocrystals with trans-1,2-bis(2-pyridyl)ethylene (2,2′-bpe)
assembled by N···I X-bonds. In each cocrystal, 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-tFb)·(2,2′-
bpe), the donor molecules support the C=C bonds of 2,2′-bpe to undergo an intermolecular [2+2]
photodimerization. UV irradiation of each cocrystal resulted in stereospecific and quantitative
conversion of 2,2′-bpe to rctt-tetrakis(2-pyridyl)cyclobutane (2,2′-tpcb). In each case, the reactivity
occurs via face-to-face π-stacked columns wherein nearest-neighbor pairs of 2,2′-bpe molecules lie
sandwiched between X-bond donor molecules. Nearest-neighbor C=C bonds are stacked criss-
crossed in both cocrystals. The reactivity was ascribed to the olefins undergoing pedal-like motion
in the solid state. The stereochemistry of 2,2′-tpcb is confirmed in cocrystals 2(1,3-di-I-tFb)·(2,2′-
tpcb) and (1,4-di-I-tFb)·(2,2′-tpcb).
Keywords: cocrystal; crystal engineering; halogen bonding; photodimerization; cyclobutane; pedal
motion.
1. Introduction
Halogen bonding (X-bonding) is a well-established [1] supramolecular synthon, as this
directional, noncovalent force can reliably direct the self-assembly of molecules into supramolecular
architectures [2,3]. Electrophilic regions associated with halogen (X) atoms that serve as X-bond
donors recognize complementary, nucleophilic (:Nu) functionalities (e.g., N-atoms) on acceptor
molecules such that an X-bond, a special class of Lewis acid-base adduct, X···Nu, results [4].
Due to the highly directional character of X-bonds, X-bond donor molecules with rigid
geometries have attracted much interest in the field of crystal engineering. A necessary, albeit
admittedly ambitious, aim is to achieve a functional level of predictive power in the design of desired
geometries and/or topologies based principally on geometries of the X-bond donor and acceptor
molecules [2,3]. Successes would be invaluable in the design of functional materials and would
constitute a major accomplishment for the fields of crystal engineering and supramolecular
chemistry.
In this context, an application of X-bonding of interest to our group is that of exploiting X-bonds
as supramolecular synthons to design photoreactive solids. We have devoted efforts to investigate
whether multicomponent systems can be assembled into predictable geometries based primarily on
the directional character of X-bonds and molecular structures of the components. Ideally, judicious
choice of coformers would enable access to solids of functional significance, such as reactivity [5–9].
Molecules 2020, 25, 907; doi:10.3390/molecules25040907
www.mdpi.com/journal/molecules

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Perfluorinated iodoarenes are well-established X-bond donors and are currently among the most
frequently employed building blocks in the crystal engineering of X-bonded materials. In the context
of systematic, geometry-based design, such donors are attractive owing to their planar and rigid
structure as well as the well-established enhancement of X-bond donor capability by poly-
fluorosubstitution [10]. We very recently described [11] that the cocrystallization of 1,2-
diiodotetrafluorobenzene (1,2-di-I-tFb) with trans-1,2-bis(n-pyridyl)ethylene (n,n’-bpe, where: n = n’
= 2, 3, 4) affords cocrystals sustained by N···I X-bonds that make up one-dimensional (1D) and two-
dimensional (2D) assemblies. For 2,2′-bpe, the components generated a discrete aggregate packed as
a photostable 1D assembly.
Among the structural isomers of diiodotetrafluorobenzene, the most ubiquitous X-bond donor
is 1,4-di-I-tFb, with a far less common donor being 1,3-di-I-tFb (Scheme 1). A survey of the
Cambridge Structural Database (CSD) [12] revealed 146 structures that involve 1,4-di-I-tFb as an X-
bond donor, whereas only five structures exist with 1,3-di-I-tFb as the donor.
Scheme 1. Components for cocrystals.
Herein, we report cocrystals involving 2,2′-bpe with 1,3-di-I-tFb and 1,4-di-I-tFb as X-bond
donors (Scheme 1). The components of each cocrystal, 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-
tFb)·(2,2′-bpe), assembled into 1D chains mediated by N···I X-bonds. Adjacent chains interacted via
face-to-face π-stacking interactions such that two molecules of 2,2′-bpe lay sandwiched between X-
bond donors. We show the arrangements that enabled each sandwiched, nearest-neighbor pair of
2,2′-bpe to undergo an intermolecular [2+2] photodimerization to furnish rctt-tetrakis(2-
pyridyl)cyclobutane (2,2′-tpcb) quantitatively and stereospecifically (Scheme 2). The solid-state
reactivity occurred despite the major fraction of nearest-neighbor C=C bonds of 2,2′-bpe laying
stacked criss-crossed in the solids. We ascribed the reactivity to the C=C bonds undergoing solid-
state pedal-like motion [13-22]. The photoproduct was confirmed in the X-bonded cocrystals 2(1,3-
di-I-tFb)·(2,2′-tpcb) and (1,4-di-I-tFb)·(2,2′-tpcb).
Scheme 2. UV-irradiation of 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-tFb)·(2,2′-bpe) to form 2,2′-tpcb.

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2. Results and Discussion
2.1. X-ray Structure of 2(1,3-di-I-tFb)·2(2,2′-bpe)
The components of 2(1,3-di-I-tFb)·2(2,2′-bpe) crystallized in the orthorhombic,
noncentrosymmetric space group Pna21 (Table 1). The asymmetric unit consists of two molecules of
1,3-di-I-tFb and two molecules of 2,2′-bpe that assemble by a combination of N···I X-bonds (d(I1···N1)
= 2.96(2) Å; d(I2···N2) = 3.00(1) Å; d(I3···N3) = 2.99(1) Å; d(I4···N4) = 3.04(1) Å) (Table 2), as well as
edge-to-edge C-H···I and C-H···F contacts (Figure 1a, Table 3). The pyridyl rings of one molecule of
2,2′-bpe (N1/N2) lie perfectly coplanar (twist angle = 0.00°), whereas the pyridyl rings of the other
(N3/N4) deviate from coplanarity (twist angle ~ 5.35°). Both molecules of 2,2′-bpe adopt an anti-
conformation (Figure 1a). The C=C bonds of each molecule of 2,2′-bpe lie disordered over two sites
(occupancies: 72/28 [N1/N2]; 72/28 [N3/N4]). The major fraction (72%) of nearest-neighbor C=C bonds
lie criss-crossed [13].
Table 1. Crystallographic data and structure refinement statistics.*.
Cocrystal
CCDC deposition number
Empirical formula
Formula weight/g·mol-1
Temperature/K
Crystal system
2(1,3-di-I-tFb)·2(2,2’-bpe)
1940759
C18H10F4I2N2
584.08
296.15
orthorhombic
2(1,3-di-I-tFb)·(2,2’-tpcb)
1967637
C36H20F8I4N4
1168.16
150.15
monoclinic
(1,4-di-I-tFb)·(2,2’-bpe)
1940762
C18H10F4I2N2
584.08
296.15
tetragonal
(1,4-di-I-tFb)·(2,2’-tpcb)
1967634
C30H20F4I2N4
766.3
296.15
monoclinic
Space group
Pna
P
I
cd
41
C
c
2/
21
21/c
a
b
c /Å
α /°
β /°
γ /°
/Å
/Å
21.975(2)
19.2247(19)
9.0190(9)
90
90
90
3810.2(6)
8
2.036
3.342
2192
0.32 × 0.23 × 0.22
MoKα (λ = 0.71073)
9.3925(9)
18.8849(19)
20.596(2)
90
90.337(5)
90
3653.2(6)
4
2.124
3.486
2192
0.205 × 0.145 × 0.03
MoKα (λ = 0.71073)
4.844 to 52.798
14.1404(14)
14.1404(14)
38.138(4)
90
90
90
7625.7(17)
16
2.035
3.34
4384
0.23 × 0.19 × 0.045
MoKα (λ = 0.71073)
5.762 to 53.438
25.422(3)
5.4525(5)
21.183(2)
90
112.099(5)
90
2720.5(5)
4
1.871
2.367
1480
0.16 × 0.16 × 0.05
MoKα (λ = 0.71073)
6.326 to 53.426
Volume/Å3
Z
ρ
calc/g·cm-3
μ /mm-1
F (000)
Crystal size/mm3
Radiation
2Θ range for data collection/° 3.706 to 53.308
Index ranges
-27 ≤ h ≤ 27, -24 ≤ k ≤ 23, -11 ≤ l ≤ 11 -11 ≤ h ≤ 11, -23 ≤ k ≤ 23, -25 ≤ l ≤ 25 -17 ≤ h ≤ 17, -17 ≤ k ≤ 17, -48 ≤ l ≤ 41 -32 ≤ h ≤ 32, -6 ≤ k ≤ 6, -26 ≤ l ≤ 21
24085 41568 21996 7854
Reflections collected
Independent reflections
Data/restraints/parameters
R
R
R
R
R
R
R
R
7967 [ int = 0.0302, sigma = 0.0287] 7413 [ int = 0.0373, sigma = 0.0299] 3771 [ int = 0.0374, sigma = 0.0225] 2870 [ int = 0.0338, sigma = 0.0413]
7967/1/482
1.091
7413/4/498
1.153
3771/1/242
1.062
2870/1/196
1.077
Goodness-of-fit on F 2
Final R indices [I ≥ 2σ (I )]
R
1 = 0.0537
wR 2 = 0.1355
1 = 0.0685
R
1 = 0.0463
wR 2 = 0.0864
1 = 0.0613
R
1 = 0.0260
wR 2 = 0.0552
1 = 0.0394
R
1 = 0.0352
wR 2 = 0.0745
R
1 = 0.0641
R indices (all data)
R
R
R
wR 2 = 0.1522
3.50/-1.09
0.284(14)
wR 2 = 0.0926
3.41/-3.00
-
wR 2 = 0.0603
0.24/-0.34
0.254(13)
wR 2 = 0.0838
0.441/-0.676
-
Largest diff. peak/hole/e·Å-3
Flack parameter
ꢇ
ꢇ ꢇ ꢁ/ꢇ
ꢇꢊꢇꢋ ∑
ꢆ ꢃ − ꢃ / [ꢆ(ꢃ ) ]
ꢅ
*
∑ |
ꢂ ꢃ
|
|
|
∑|
|
ꢈ∑
|
ꢉ
ꢀꢁ
=
−
ꢃ ꢂ / ꢃ , ꢆꢀꢇ
=
|
.
ꢄ
ꢅ
ꢄ
ꢄ
ꢄ
Goodness-of-fit on F2 =
[
ꢆ
ꢃ
∑
(|
|)ꢇ
− ꢃ /(ꢌꢄꢍꢎ − ꢌꢏꢐꢑꢐꢒꢓꢔꢓꢑ)]ꢁ/ꢇ
.
ꢄ
ꢅ
The components formed two crystallographically-unique 1D chains wherein each molecule of
1,3-di-I-tFb bridges two molecules of 2,2′-bpe via the N···I X-bonds (Figure 1b,c). Assembly
properties of the X-bonded cocrystals 2(1,3-di-I-tFb)·2(2,2′-bpe), 2(1,3-di-I-tFb)·(2,2′-tpcb), (1,4-di-I-
tFb)·(2,2′-bpe), and (1,4-di-I-tFb)·(2,2′-tpcb) are addressed below (Table 3). The chains interact by
way of offset, face-to-face π-stacks involving 1,3-di-I-tFb of one chain and one pyridyl (pyr) ring of
2,2′-bpe of the other chain (d(pyr[N2]···1,3-di-I-tFb[I3/I4] ~ 4.48 Å; d(pyr[N4]···1,3-di-I-tFb[I1/I2] ~ 4.55 Å).
Across the π-stacked chains, two molecules of 2,2′-bpe lie sandwiched between two unique
molecules of 1,3-di-I-tFb in an ABB’A’ motif (Figure 1e, Table 3). As a consequence of this
arrangement, nearest-neighbor molecules of 2,2′-bpe stack parallel with C=C bonds separated on the
order of 3.64 Å.
Table 2. X-bond metrics for cocrystals.

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Cocrystal
X-Bond d (N···I)/Å Θ (C-I···N)/° X-Bond Type Prs*
2(1,3-di-I-tFb)·(2,2’-bpe) I1···N1
2.96(2)
3.00(1)
2.99(1)
172.64
174.16
172.24
174.42
174.39
167.06
177.44
169.48
177.09
177.16
174.36
171.42
I
I
I
I
I
I
I
I
I
I
I
I
16
15
15
14
16
15
17
11
16
16
13
17
I2···N2
I3···N3
I4···N4
3.04(1)
2(1,3-di-I-tFb)·(2,2’-tpcb) I1···N1
2.949(6)
3.009(6)
2.947(6)
3.131(6)
2.967(8)
2.982(8)
3.064(4)
2.942(4)
I2···N4
I3···N3
I4···N2
(1,4-di-I-tFb)·(2,2’-bpe) I1···N1
I2···N2
(1,4-di-I-tFb)·(2,2’-tpcb) I1···N1A
I1···N1B
*
Prs ≡ percent relative shortening = {1 - d(N···I)/[rvdW(I) + rvdW(N)]}·100, with rvdW(N) = 1.55 Å, and
rvdW(I) = 1.98 Å.
Although the C=C bonds of 2,2′-bpe adopt a criss-crossed geometry, the alkene is photoactive.
UV irradiation of a microcrystalline sample of 2(1,3-di-I-tFb)·2(2,2′-bpe) for a period of 20 h resulted
in stereospecific and quantitative conversion of 2,2′-bpe to 2,2′-tpcb. The formation of the
photoproduct was evidenced by the complete disappearance of the olefinic resonance (7.70 ppm) and
emergence of a single cyclobutane resonance (4.93 ppm) [23,24]. We note that, as a single component,
2,2′-bpe has been reported to be photostable [24].
Table 3. Structural features of cocrystals.
Cocrystal
Primary Assembly
Secondary Assembly
Photoreactivity
2(1,3-di-I-tFb)·2(2,2'-bpe) two unique, infinite 1D chains based on N···I
face-to-face ABB'A' π-stacked sandwiches active
2(1,3-di-I-tFb)·(2,2'-tpcb)
(1,4-di-I-tFb)·(2,2'-bpe)
π
-
-
infinite 1D chain based on N···I and face-to-face -stacks
infinite 1D chain based on N···I and edge-to-edge C-H···I
face-to-face ABBA π-stacked sandwiches
active
-
(1,4-di-I-tFb)·(2,2'-tpcb) infinite zig-zag chains based on N···I
herringbone pattern based on C-H···N

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Figure 1. Perspectives of 2(1,3-di-I-tFb)·2(2,2′-bpe): (a) asymmetric unit (olefin disorder omitted for
clarity); (b) 1D chain based on N1/N2 illustrating ABAB repeat unit; (c) 1D chain based on N3/N4
illustrating ABAB repeat unit; (d) offset, face-to-face π-stacks (highlighted in red) between chains; (e)
face-to-face π-stacked sandwiches (space-filling) illustrating ABB’A’ repeat unit; and (f) π-stacked
sandwiches highlighting criss-crossed C=C bonds (olefin disorder omitted for clarity).
2.2. X-ray Structure of 2(1,3-di-I-tFb)·(2,2′-tpcb)
The photoproduct 2,2′-tpcb interacts with 1,3-di-I-tFb as a tetratopic [25,26] X-bond acceptor.
The components of 2(1,3-di-I-tFb)·(2,2′-tpcb) crystallize in the monoclinic space group P2 /c (Table
1
1). The asymmetric unit consists of two unique molecules of 1,3-di-I-tFb and one molecule of 2,2′-
tpcb. The cyclobutane ring lies disordered over two sites (occupancies: 84/16 at T = 150.15 K). The
molecules of 1,3-di-I-tFb interact with each another via face-to-face π-stacks (d ~ 3.77 Å, Figure 2a).
The cyclobutane 2,2′-tpcb interacts with one molecule of 1,3-di-I-tFb [I3/I4] via two face-to-face π···F
forces involving two of the pyridyl (pyr) rings of 2,2′-tpcb (d(pyr[N1]···F5) ~ 3.71 Å; d(pyr[N4]···F6) ~ 3.42
Å, Figure 2a). As a consequence of the assembly process, the components form 1D chains via a
combination of N···I X-bonds (d(I1···N1) = 2.949(6) Å; d(I2···N4) = 3.009(6) Å; d(I3···N3) = 2.947(6) Å;
d(I4···N2) = 3.131(6) Å) (Table 2) and face-to-face π-stacks between molecules of 1,3-di-ItFb (Figure
2a,b, Table 3).

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Figure 2. Perspectives of 2(1,3-di-I-tFb)·(2,2′-tpcb): (a) asymmetric unit highlighting π···π and π···F
forces (cyclobutane disorder omitted for clarity) and (b) 1D chains (H atoms omitted for clarity).
2.3. X-ray Structure of (1,4-di-I-tFb)·(2,2′-bpe)
The components of (1,4-di-I-tFb)·(2,2′-bpe) crystallize in the noncentrosymmetric tetragonal
space group I4 cd (Table 1). The asymmetric unit consists of one molecule of 1,4-di-I-tFb and one
1
molecule of 2,2′-bpe that assemble by N···I X-bonds (d(I1···N1) = 2.967(8) Å; d(I2···N2) = 2.982(8) Å)
(Figure 3a,b, Table 2). The pyridyl rings of 2,2′-bpe lie approximately coplanar (twist angle ~ 4.83°).
The alkene 2,2′-bpe adopts an anti-conformation (Figure 3a). The C=C bonds of 2,2′-bpe lie disordered
over two sites (occupancies: 66/34). The components form infinite 1D chains along the
crystallographic bc-plane, sustained by a combination of N···I X-bonds and edge-to-edge C-H···I
forces (Table 3). In these chains, each molecule of 1,4-di-I-tFb bridges two molecules of 2,2′-bpe by
way of two N···I X-bonds such that the chains repeat in an ABAB pattern (Figure 3b, Table 3). Adjacent
chains interact by way of offset, face-to-face π-stacks involving 1,4-di-I-tFb and one pyridyl ring of
2,2′-bpe (d(pyr[N1]···1,3-di-I-tFb ~ 4.67 Å) (Figure 3c).
Similar to 2(1,3-di-I-tFb)·2(2,2′-bpe), across the π-stacked chains, two molecules of 2,2′-bpe lie
sandwiched between two molecules of 1,3-di-I-tFb in an ABBA motif (Figure 3d, Table 3). As a
consequence of this arrangement, nearest-neighbor molecules of 2,2′-bpe stack parallel with C=C
bonds separated on the order of 3.72 Å, with the major sites (occupancies: 66%) of nearest-neighbor
C=C bonds being criss-crossed [13-22]. Despite the crossed geometry, however, 2,2′-bpe is
photoactive. UV irradiation of a microcrystalline sample of (1,4-di-I-tFb)·(2,2′-bpe) for a period of 20
h resulted in stereospecific and quantitative conversion of 2,2′-bpe to 2,2′-tpcb. Formation of the
photoproduct was evidenced by complete disappearance of the olefinic resonance (7.70 ppm) and
emergence of a single cyclobutane resonance (4.93 ppm) [23,24].

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Figure 3. Perspectives of (1,4-di-I-tFb)·(2,2′-bpe): (a) asymmetric unit; (b) 1D chain along
crystallographic bc-plane illustrating the ABAB repeat unit; (c) offset, face-to-face π-stacks
(highlighted in red) between chains; (d) face-to-face π-stacked sandwiches (space-filling) illustrating
the ABBA repeat unit; and (e) π-stacked sandwiches highlighting criss-crossed C=C bonds (olefin
disorder omitted for clarity).
2.4. X-ray Structure of (1,4-di-I-tFb)·(2,2′-tpcb)
The components of (1,4-di-I-tFb)·(2,2′-tpcb) crystallize in the monoclinic space group C2/c (Table
1). The asymmetric unit consists of one-half molecule of 1,4-di-I-tFb, which lies on a crystallographic
center of inversion, and one-half molecule of 2,2′-tpcb which lies on a two-fold axis and is disordered
(occupancies: 50/50) by symmetry (Figure 4a). The components of (1,4-di-I-tFb)·(2,2′-tpcb) assemble
primarily via N···I X-bonds (d(I1···N1A) = 3.064(4) Å; d(I1···N1B) = 2.942(4) Å) to generate infinite zig-
zag chains (λ ~ 2.6 nm, Θ ~ 81.6°) (Figure 4b, Table 3). Adjacent chains interact via C-H···N contacts
between pyridyl rings (d(C7-H7···N2) = 3.449(8) Å) to generate a herringbone pattern (Figure 4c, Table
3). Adjacent molecules of 2,2′-tpcb also interact via edge-to-face C-H···π forces (d(C14-H14···pyr[N1]) ~
4.15 Å) and offset, face-to-face π···π stacks (d(pyr[N2]···pyr[N2]) ~ 4.51 Å) to generate infinite 1D chains
along the crystallographic c-axis (Figure 4d).

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Figure 4. Perspectives of (1,4-di-I-tFb)·(2,2′-tpcb): (a) asymmetric unit (cyclobutane disorder omitted
for clarity); (b) infinite zig-zag chains (space-filling, view along c); (c) herringbone pattern resulting
from interactions between zig-zag chains; and (d) 1D chains of 2,2′-tpcb generated along the
crystallographic c-axis, highlighting offset, face-to-face π···π forces.
2.5. Photoreactivity
We very recently invoked pedal-like motion to account for the stereospecific and quantitative
[2+2] photodimerization of 3,3′-bpe to form rctt-3,3′-tpcb in the hydrogen-bonded cocrystal
(catechol)·(3,3′-bpe) [10]. The reactivity occurred between criss-crossed C=C bonds. Pedal motion can
also be used to explain the photodimerizations of cocrystals 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-
tFb)·(2,2′-bpe) to form rctt-2,2′-tpcb. The C=C bonds of the two cocrystals also exhibited a criss-
crossed geometry that, in contrast to (catechol)·(3,3′-bpe), also lie disordered. A disordered geometry
is typically associated with pedal motion that supports reactivity [11,13-22].
3. Materials and Methods
3.1. General Experimental
All reagents and solvents (synthesis grade) were purchased from commercial sources and used
as received unless otherwise stated. 1,3-diiodotetrafluorobenzene (1,3-di-I-tFb) was purchased from
Apollo Scientific; 1,4-diiodotetrafluorobenzene (1,4-di-I-tFb) was purchased from Aldrich; trans-1,2-
bis(2-pyridyl)ethylene (2,2′-bpe) was purchased from TCI America; and chloroform (CHCl ; certified
3
ACS grade, ≥99.8%, approximately 0.75% EtOH as preservative) was purchased from Fisher
Chemical. All cocrystal syntheses were conducted in screw-cap glass scintillation vials. For cocrystal
syntheses, “thermal dissolution” refers to the process of combining both solid cocrystal components
in a vial, adding solvent portion-wise while maintaining a saturated mixture at room temperature
(rt), and tightly capping the vial and heating the mixture on a hot-plate until all solids dissolve to
afford a homogeneous solution with the minimum necessary volume of solvent. Compositions of all
single crystals were shown to be representative of the bulk material by matching experimental
powder X-ray diffraction (pXRD) patterns with those simulated from single-crystal X-ray diffraction
(scXRD) data. Yields refer to isolated yields of analytically-pure compounds unless otherwise stated.
Melting points (mp) were recorded on samples in open capillary tubes using a manual MEL-TEMP
apparatus (Electrothermal Corporation, Staffordshire, UK) and were uncorrected. Photoreactions
were conducted in either a NuLink 36 W UV lamp apparatus (λ = 400 nm) or an ACE photocabinet

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equipped with an ACE quartz, 450 W, broadband, medium pressure, Hg-vapor lamp. Photoreactions
were conducted by: (1) grinding single crystals of the cocrystal to a fine powder with an agate mortar
and pestle; (2) smearing the powder between two Pyrex plates; and (3) irradiating the powder in 10
h intervals, taking care to ensure uniform irradiation.
3.2. Synthetic Procedures
2(1,3-di-I-tFb)·2(2,2′-bpe). Note: 2(1,3-di-I-tFb)·2(2,2′-bpe) is photosensitive. Cocrystals of 2(1,3-
di-I-tFb)·2(2,2′-bpe) were obtained by thermal dissolution of 2,2′-bpe (85.3 mg, 0.459 mmol) and 1,3-
di-I-tFb (182.4 mg, 0.440 mmol, 0.96 equiv) in CHCl3 (2.0 mL). Upon cooling to rt, single crystals of
2(1,3-di-I-tFb)·2(2,2′-bpe)—colorless blocks, suitable for scXRD—formed within 6 d. Analytical data:
1
mp 90-91 °C (CHCl3). H NMR (400 MHz, DMSO-d6): δ 8.61 (ddd, J = 4.8, 1.8, 0.9 Hz, 4Ha), 7.82 (app
td, J = 7.7, 1.8 Hz, 4Hb), 7.70 (s, 4Hc), 7.63 (app dt, J = 7.8, 1.0 Hz, 4Hd), 7.31 (ddd, J = 7.5, 4.8, 1.1 Hz,
4He).
2(1,3-di-I-tFb)·(2,2′-tpcb). Single crystals of 2(1,3-di-I-tFb)·2(2,2′-bpe) were ground to a fine
powder with an agate mortar and pestle and smeared between two Pyrex plates. The plate assembly
1
was placed in an ACE photocabinet. After 20 h, H NMR assay revealed quantitative and
stereospecific conversion to 2(1,3-di-I-tFb)·(2,2′-tpcb). The product powder was scraped from the
plates, dissolved in CHCl3 at rt, and allowed to slowly evaporate. Single crystals of 2(1,3-di-I-
tFb)·(2,2′-tpcb)—colorless plates, suitable for scXRD—formed within 9 d. Analytical data: mp 112-
117 °C, 154–156 °C (dec., CHCl3) (two distinct melting points were observed: presumably, one for
each of the two coformers. Only the second melting point range (154–156 °C) was accompanied by a
1
noticeable decomposition). H NMR (400 MHz, DMSO-d6): δ 8.34 (ddd, J = 4.8, 1.8, 0.8 Hz, 4Ha), 7.46
(app td, J = 7.7, 1.8 Hz, 4Hb), 7.09 (d, J = 7.8 Hz, 4Hc), 6.99 (ddd, J = 7.4, 4.8, 1.1 Hz, 4Hd), 4.93 (s, 4He).
(1,4-di-I-tFb)·(2,2′-bpe). Note: (1,4-di-I-tFb)·(2,2′-bpe) is photosensitive. Cocrystals of (1,4-di-I-
tFb)·(2,2′-bpe) were obtained by thermal dissolution of 2,2′-bpe (92.1 mg, 0.495 mmol) and 1,4-di-I-
tFb (208.6 mg, 0.509 mmol, 1.03 equiv) in CHCl3 (2.0 mL). Upon cooling to rt, single crystals of (1,4-
di-I-tFb)·(2,2′-bpe)—colorless plates, suitable for scXRD—formed within 24 h. Analytical data: mp
1
155-156 °C (dec., CHCl3). H NMR (400 MHz, DMSO-d6): δ 8.61 (ddd, J = 4.8, 1.8, 0.9 Hz, 4Ha), 7.82
(app td, J = 7.7, 1.8 Hz, 4Hb), 7.70 (s, 4Hc), 7.63 (app dt, J = 7.8, 1.0 Hz, 4Hd), 7.31 (ddd, J = 7.5, 4.8, 1.1
Hz, 4He).
2(1,4-di-I-tFb)·(2,2′-tpcb). Single crystals of 2(1,4-di-I-tFb)·2(2,2′-bpe) were ground to a fine
powder with an agate mortar and pestle and smeared between two Pyrex plates. The plate assembly
1
was placed in an ACE photocabinet. After 20 h, H NMR assay revealed quantitative and
1
stereospecific conversion to 2(1,4-di-I-tFb)·(2,2′-tpcb). Analytical data: H NMR (400 MHz, DMSO-
d6): δ 8.34 (ddd, J = 4.8, 1.8, 0.8 Hz, 4Ha), 7.46 (app td, J = 7.7, 1.8 Hz, 4Hb), 7.09 (d, J = 7.8 Hz, 4Hc), 6.99
(ddd, J = 7.4, 4.8, 1.1 Hz, 4Hd), 4.93 (s, 4He).
(1,4-di-I-tFb)·(2,2′-tpcb). The product powder, 2(1,4-di-I-tFb)·(2,2′-tpcb), was scraped from the
plate assembly, dissolved in CHCl3 at rt, and allowed to slowly evaporate. Single crystals of (1,4-di-
I-tFb)·(2,2′-tpcb)—colorless plates, suitable for scXRD—formed within 6 d. Analytical data: mp 174-
175 °C (dec., CHCl3).
1
3.3. H NMR Spectroscopy
1
Proton nuclear magnetic resonance ( H NMR) spectra were recorded at room temperature on a
Bruker DRX-400 spectrometer at 400 MHz (instrument parameters: field strength: 9.2 T; RF-Console:
1
15
31
DRX 3-channel; magnet: shielded superconducting; probe: nature, 5.0 mm BBO- H/ N- P; type,
1
double-resonance; temperature range, -100–180 °C). H NMR data are reported as follows: chemical
shift (δ, ppm), multiplicity (s = singlet, d = doublet, ddd = doublet of doublet of doublets, app dt =
apparent doublet of triplets, app td = apparent triplet of doublets), coupling constant(s) (J, Hz), and
integration. Chemical shift values were calibrated relative to residual solvent resonance (DMSO: δH =
2.50 ppm) as the internal standard. All NMR data were collected and plotted within the Bruker
TopSpin v4.0.6 software suite.

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3.4. Powder X-ray Diffraction (pXRD)
Powder X-ray diffraction data were collected at room temperature on a Bruker D8 Advance X-
ray diffractometer on samples mounted on glass slides. Each sample was finely ground using an
agate mortar and pestle prior to mounting. Instrument parameters: radiation wavelength, CuKα (λ =
1.5418 Å); scan type, coupled TwoTheta/Theta; scan mode, continuous PSD fast; scan range, 5–40°
two-theta; step size, 0.02°; voltage, 40 kV; current, 30 mA. Background subtractions were applied to
all experimentally collected data within the Bruker DIFFRAC.EVA v3.1 software suite. All data were
plotted in Microsoft Excel 2016. Simulated pXRD patterns were calculated from scXRD data within
the Cambridge Crystallographic Data Centre (CCDC) Mercury [27] software suite.
3.5. Single-Crystal X-ray Diffraction (scXRD)
Single-crystal X-ray diffraction data were collected on either a Bruker Nonius-Kappa APEX II
CCD or a Bruker Nonius-Kappa CCD diffractometer, each equipped with an Oxford Cryosystems
700 series cold N2 gas stream cooling system. Data were collected at either room temperature (298.15
K) or low temperature (150.15 K) using graphite-monochromated MoKα radiation (λ = 0.71073 Å).
Crystals were mounted in Paratone oil on a MiTeGen magnetic mount. Data collection strategies for
TM
ensuring maximum data redundancy and completeness were calculated using the Bruker Apex II
software suite. Data collection, initial indexing, frame integration, Lorentz-polarization corrections,
and final cell parameter calculations were likewise accomplished using the Apex II software suite.
Multi-scan absorption corrections were performed using SADABS [28]. Structure solution and
refinement were accomplished using SHELXT [29] and SHELXL [30], respectively, within the Olex2
[31] graphical user interface. Space groups were unambiguously verified using the PLATON [32]
executable. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were attached
via a riding model at calculated positions using suitable HFIX commands. The occupancies of the
major and minor positions for the disordered C=C cores within 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-
di-I-tFb)·(2,2′-bpe) and for the disordered cyclobutane C-C cores within 2(1,3-di-I-tFb)·(2,2′-tpcb)
and (1,4-di-I-tFb)·(2,2′-tpcb) converged to their respective ratios after each was identified in the
difference map and freely refined. Figures of all structures were rendered in the CCDC Mercury [27]
software suite.
4. Conclusions
We demonstrated that X-bonds support intermolecular [2+2] photodimerizations of 2,2‘-bpe in
the solid state. The reaction occurred stereospecifically and quantitatively to generate 2,2′-tpcb. The
cocrystals 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-tFb)·(2,2′-bpe) provide mounting evidence that X-
bonds can be utilized in a more general way to sustain the formation of C-C bonds in solids. Our
efforts are now focused on rational approaches to utilize X-bonds to direct stacking and control
reactivity in the solid state.
Supplementary Materials: The following are available online, Figure S1: 1H NMR spectrum of 2,2′-bpe, Figure
1
1
S2: H NMR spectrum of 2(1,3-di-I-tFb)·(2,2′-tpcb), Figure S3: H NMR spectrum of 2(1,4-di-I-tFb)·(2,2′-tpcb),
Figure S4: pXRD pattern of 2(1,3-di-I-tFb)·2(2,2′-bpe), Figure S5: pXRD pattern of 2(1,3-di-I-tFb)·(2,2′-tpcb),
Figure S6: pXRD pattern of (1,4-di-I-tFb)·(2,2′-bpe), Figure S7: pXRD pattern of (1,4-di-I-tFb)·(2,2′-tpcb), Tables
S1–S4: Crystallographic data and structure refinement statistics for 2(1,3-di-I-tFb)·2(2,2′-bpe), 2(1,3-di-I-
tFb)·(2,2′-tpcb), (1,4-di-I-tFb)·(2,2′-bpe), and (1,4-di-I-tFb)·(2,2′-tpcb), Table S5: Bond metrics for cocrystals,
Table S6: Melting point data for cocrystals and coformers thereof.
Author Contributions: All authors have read and agree to the published version of the manuscript.
Conceptualization, J.Q. and L.R.M.; methodology, J.Q.; formal analysis, J.Q., D.C.S., and L.R.M.; investigation,
J.Q. and L.R.M.; resources, L.R.M.; writing—original draft preparation, J.Q. and L.R.M.; writing—review and
editing, J.Q. and L.R.M.; supervision, L.R.M.; funding acquisition, L.R.M. All authors have read and agreed to
the published version of the manuscript.
Funding: This research was funded by the National Science Foundation, grant number LRM DMR-1708673.

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Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to
publish the results.
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Sample Availability: Samples of the compounds 2(1,3-di-I-tFb)·2(2,2′-bpe), (1,4-di-I-tFb)·(2,2′-bpe), 2(1,3-di-I-
tFb)·(2,2′-tpcb), and (1,4-di-I-tFb)·(2,2′-tpcb) are available from the authors.
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article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).